Nvidia didn't start allowing you to run more than 2 displays on a single card until the 600 series, with the exception of dual GPU cards like the GTX 590. If you want to run three monitors, you'll have to either get a second 560Ti and run them in SLI (if your motherboard supports it), or buy a new video card (any 600 series Geforce card, or any AMD Radeon Card)